1. Field of the Invention
This invention relates to an improved process and apparatus for producing sodium hydrosulfite. In particular, improved sodium hydrosulfite yields are obtained when contrasted to the prior art.
Sodium hydrosulfite, Na.sub.2 S.sub.2 O.sub.4, also known as sodium dithionite, is extensively used as a bleaching agent in the paper and textile industries, and has a wide range of other uses. Because it is relatively unstable, it is generally produced in situ at the point of use, for example in a pulp mill.
2. Description of the Prior Art
Methods used in the past for producing sodium hydrosulfite have included dissolving zinc in a solution of sodium bisulfite and precipitating zinc-sodium sulfite with milk of lime to leave the hydrosulfite in solution, and reacting sodium formate with sodium hydroxide and sulfur dioxide.
More recent processes include mixing caustic soda and sulfur dioxide with sodium borohydride in an aqueous medium to produce an aqueous solution of sodium hydrosulfite. The sodium borohydride generally enters the process in a mixture with aqueous sodium hydroxide. This mixture, obtainable from the Ventron Division of Morton Thiokol, Inc. under the registered trademark "BOROL", has good stability because acid hydrolysis of the sodium borohydride is prevented. The first mixture typically comprises 10-15 wt % sodium borohydride, 35-45 wt % sodium hydroxide, and 40-55 wt % water. A typical mixture comprises 12 wt % sodium borohydride, 40 wt % sodium hydroxide, and 48 wt % water. For convenience, this type of process will be referred to hereinafter as the BOROL process.
The theoretical reaction of the BOROL process, assuming ideal conditions and 100% yield, would be as follows:
NaBH.sub.4 +8NaOH+8SO.sub.2 .fwdarw.4Na.sub.2 S.sub.2 O.sub.4 +NaBO.sub.2 +6H.sub.2 O
There is, however, a side reaction in which the sodium borohydride is hydrolyzed, thus reducing the overall efficiency of the reaction: ##STR1##
This reaction is a function of pH and increases with reduced pH. The problem cannot, however, be overcome simply by raising the pH since this would adversely affect the main reaction. The reaction effectively takes place in two stages, as follows:
(a) the reaction between sulfur dioxide and caustic soda to give sodium bisulfite (I); and
(b) the reaction between the bisulfite and sodium borohydride to give sodium hydrosulfite (II). EQU 8NaOH+8SO.sub.2 .fwdarw.8NaHSO.sub.3 (I) EQU 8NaHSO.sub.3 +NaBH.sub.4 .fwdarw.4Na.sub.2 S.sub.2 O.sub.4 +NaBO.sub.2 +6H.sub.2 O (II)
There is also an equilibrium (III) between the bisulfite and sodium sulfite, which is a function of the pH: ##STR2## K.sub.2 =1.02.times.10.sup.-7 (18.degree. C.) K.sub.1 =1.54.times.10.sup.-2 (18.degree. C.)
Above pH7, the bisulfite concentration is inversely proportional to pH. Below pH2, the bisulfite concentration is directly proportional to pH. In the pH range 5-7, within which this type of process is generally operated, lowering the pH will favour the formation of bisulfite.
Consideration of this equilibrium, therefore, has to be weighed against that of acid hydrolysis discussed above to determine the optimum pH for the process. In the process used hitherto a pH of 6.5 has been found to give the best yield. Nevertheless, it has proved difficult to achieve yields greater than about 85%.
In the sodium hydrosulfite generation process used hitherto, SO.sub.2, water, sodium hydroxide (NaOH), and a sodium borohydride/sodium hydroxide/water mixture (BOROL) are fed in that order into a flow line which leads to a static mixer and thence to a degassing tank where entrained gases are vented to the atmosphere. An aqueous solution of sodium hydrosulfite is pumped from the degassing tank, part of this being delivered to a storage tank for use as required and the rest is recycled to the flow line at a position downstream of the SO.sub.2, water and NaOH inlets but upstream of the BOROL mixture inlet. The input of each reactant can be controlled automatically in response to rising or falling levels in the degassing tank or the storage tank or changes in pressure, flow rates, and/or pH.
In addition, U.S. Pat. No. 4,788,041 depicts an improvement to the above-discussed process. This improvement obtains higher sodium hydrosulfite yields through variation in proportions of chemicals, pH measurement and control, temperature measurement and control, and specific changes in the recirculation system.
The present invention also obtains higher yields of sodium hydrosulfite but utilizes an inverse order of addition of raw materials to achieve such desirable improvement. This permits easy retrofitting of existing systems.
The present invention also obtains higher yields of sodium hydrosulfite utilizing a two compartment type mixer as depicted in FIGS. 5 and 9 in an overall system having the old order of addition of raw materials as in FIG. 2 (see Example 3), but also includes one having an inverse order of addition of such raw materials as in FIG. 5 (see Example 4).